One of the joint applicants of the present application, STEP TECHINICA Co. Ltd., filed a patent application relating to “an electronic wired system based on automatic cyclical communication” which has been granted as a patent under No. 2,994,589. This patent will be described with reference to FIG. 1. This electronic wired system is constituted by N sets of terminal devices 2, a central unit 1 for controlling the terminal devices 2 and a common communication line 3 connecting these together. Data at an input port 21 and an output port 22 of each terminal device 2 is circulated through the terminal devices in the form of packets and stored in a memory 4 of the central unit 1 at a respective one of memory locations having addresses which correspond to the terminal devices, respectively. In this electronic wired system, since transmission and reception of data is performed by means only of hardware called “a state machine”, the speed of transmission and reception of data has been significantly increased as compared to that according to the conventional communication based on a programmed control with a microprocessor. Furthermore, since data at the input port 21 and the output port 22 of each terminal device is stored in the memory, the central unit 1 can read/write data at the output/input port of any terminal device substantially in real time.
In the case of this electronic wired system, however, there is a problem that a given terminal device cannot read data from the input/output port of another terminal device or write data into such port, although the central unit 1 can read/write data at the output/input port of any terminal device as described above. The feature that a given terminal device can read data from and/or write data into another terminal device is needed in the case where a system must operate as a whole with its terminal devices acting one upon the other, such as in the case of a man-like robot or the like where a terminal device is provide for each joint.
Furthermore, there is another problem in the above-described electronic wired system that when the central unit 1 fails the entire system stops functioning since no transmission and reception of packets is performed.